Multiple systems use a magnetometer, also known as a compass, to measure Earth's magnetic field. Such systems include 9 directions of freedom (DOF) Inertial Measurement Units (IMU), Attitude and Heading Reference Systems (AHRS), Compassing Systems, and Inertial Navigation Systems (INS). These systems generally measure a strength and a direction of the Earth's magnetic field and use the results of these measurements as critical parts of providing their core functionality.
For accurate results to be produced, each system's magnetic sensing elements must be calibrated to account for magnetic distortions that affect the sensed magnetic field. These magnetic distortions are generally considered to be one of two types: hard-iron distortions or soft-iron distortions. Hard-iron distortions are caused by objects that produce a magnetic field such as a nearby current carrying conductor or a permanent magnet in a nearby component such as a speaker or a microphone. When magnetic material is part of the system and physically attached to a same reference frame as a magnetic sensing element, then a permanent bias in output from the magnetic sensing element is created. Soft-iron distortions are caused by objects that, by being present, passively distort a reference magnetic field. The presence of these soft-iron objects causes the Earth's magnetic field to appear to be stretched, bent, or otherwise distorted. Soft-iron effects vary depending upon which direction the Earth's magnetic field is oriented relative to a magnetic sensing element body itself. Soft-iron distortion is often caused by a presence of nearby ferrous metal objects and structures made of materials such as iron, steel, or nickel. Hard-iron distortions will generally have a much larger impact upon a total error than soft-iron effects.
Methods exist for performing a calibration process, which is used to determine a nature of error effects within a system and then to compute parameters to be used in a corrective step as the magnetic sensing element operates. In this way, a magnetic sensing system can be calibrated to correct for both hard-iron and soft-iron effects. Since such systems are often not located within either fixed locations or unchanging environments, such systems are often subject to external effects that can cause changes to a magnetic domain in which the magnetic sensing element is situated. The magnetic sensing system may move through a magnetic environment in which the Earth's magnetic field appears to be distorted or an external object which causes a distortion may move into the magnetic sensing system's environment. In this manner, the magnetic sensing element, having been calibrated for an original magnetic environment, will experience erroneous sensor readings due to an altered magnetic environment.